1. Field of the Invention
The present invention relates generally to a variable valve timing arrangement for an internal combustion engine and more specifically to a rocker arm construction for such an arrangement.
2. Description of the Prior Art
JU-A-63-45521 discloses the prior art arrangement shown in FIG. 14. In this arrangement a single inlet valve 101 is operatively connected with low speed/high speed dual cam arrangement by way of a rocker arm wherein said second cam follower moves with respect to said rocker arm solely in translation 104. The rocker arm 104 in this instance is of the center pivot type and includes a low speed cam follower not shown and a high speed cam follower 105. The high speed cam follower 105 is formed on the end of a cylindrical member which is reciprocatively disposed in bore formed proximate one end of the rocker arm. A spring which is disposed in the bore biases the speed cam follower 105 into engagement with the high speed cam 103.
A rotatable stopper 106 which is arranged in the bore is connected with an hydraulically operated actuator 107. In a first rotational position the stopper 106 permits the reciprocation of the cylindrical member within the bore, while in a second rotational position reciprocation is prevented.
The supply of hydraulic fluid to the actuator is determined by a microprocessor 109 which controls a switching valve 110. The valve 110 selectively supplies the output of a pump 111 via a passage structure which is formed in both the rocker shaft on which the rocker arm is pivotably mounted and the rocker arm per se.
When the actuator 107 is supplied with hydraulic fluid from the pump 111, a link 112 rotates the stopper to a position in which the locks the cam follower 105 in position and thus enables the high speed cam 103 to actuate the rocker arm.
However, this arrangement suffers from the drawback that it requires a larger number of moving parts and has a relatively large mass and resulting moment. It therefore exhibits deteriorated cam following characteristics. In addition to this, the actuator--link--stopper arrangement is relatively cumbersome and slow acting and the amount of time available during high speed engine operation, for the actuator 107 to rotate the stopper 106 to a position wherein relative movement between the rocker arm 104 and the cam follower 105 is prevented, is often insufficient for the stopper to be properly positioned.
JP-A-63-167016 and JP-A-63-57805 disclosed rocker arm arrangements which include a first rocker arm which cooperates with a low speed cam and a second rocker arm which cooperates with a high speed cam. The two rocker arms pivotally mounted on a common rocker arm shaft.
A hydraulically operated connection device which enables the first and second rocker arms to be selectively locked together, comprises a set of plunger bores which are formed in the rocker arms in a manner to be parallel with and at a predetermined distance from, the axis of the shaft about which the arms are commonly pivotal. By applying a hydraulic pressure to the end or ends of the plungers reciprocally disposed in the bores, the plungers can be induced to move axially along their bores and induce the situation wherein two of the plungers will partially enter an adjacent bore and lock the two arms together.
However, this arrangement has suffered from the drawbacks that as the rocker arms are pivotally mounted on a rocker arm shaft, minor variations in the rocker arm dimensions lead to variations in the opening and closing timing of the engine valves and in that the rocker arms become relatively large and exhibit large moments.
In addition to this, seats for the lost motion springs which are operatively connected with the high speed rocker arms must be provided on the cylinder head. This of course increases the complexity of forming and arranging the upper surface of the cylinder head.